Mixer drum driving device

ABSTRACT

A mixer drum driving device includes a mixer drum, a hydraulic motor, a hydraulic pump, an auxiliary hydraulic pump for rotating the mixer drum for mixing by supplying pressure oil to the hydraulic motor independently of the hydraulic pump, and a direct-current brush motor for driving and rotating the auxiliary hydraulic pump. The auxiliary hydraulic pump is driven and rotated by the direct-current brush motor when the engine stops during the rotation of the mixer drum for mixing.

TECHNICAL FIELD

The present invention relates to a mixer drum driving device.

BACKGROUND ART

A mixer truck carries mortar, ready-mixed concrete and the like(hereinafter, referred to as “fresh concrete”) in a mixer drum rotatablymounted on a chassis and transports these from a fresh concrete factoryto a construction site.

The mixer truck sets the mixer drum constantly in positive rotation toprevent quality degradation and solidification of the fresh concrete intransporting the fresh concrete. By the positive rotation of the mixerdrum, a plurality of spiral blades mounted in the mixer drum constantlykeeps mixing the fresh concrete. Further, the mixer truck can dischargethe fresh concrete in the mixer drum by setting the mixer drum innegative rotation in a direction opposite to the positive rotation. Themixer truck supplies the fresh concrete to a placement location bysetting the mixer drum in negative rotation upon reaching a concreteplacement site.

As just described, the mixer truck needs to constantly rotate the mixerdrum until the fresh concrete is discharged after being poured into themixer drum. An engine of the mixer truck is generally used as a drivesource used for the rotation of the mixer drum. Specifically, therotational power of the engine is transmitted to a hydraulic pump via aPTO (Power Take Off), pressure oil discharged from the hydraulic pump issupplied to a hydraulic motor and the mixer drum is driven and rotatedby the rotation of the hydraulic motor.

In a mixer drum driving device for driving a mixer drum only by anengine, the rotation speed of the engine needs to be increasedparticularly in the case of rotating the mixer drum at a high speed. Ifthe rotation speed of the engine is increased, noise is generated andthe amount of fuel consumption increases.

Further, since the mixer drum needs to be constantly kept in rotationfor reasons such as the prevention of solidification while freshconcrete is carried in the mixer drum, the engine cannot be stopped.Thus, even if the mixer truck arrives at a placement site, the engineneeds to continue to be driven although a mixer truck is in park if themixer truck is waiting to discharge the fresh concrete.

Accordingly, JP2007-278430A discloses a mixer drum driving device fordriving and rotating a mixer drum by driving an auxiliary hydraulic pumpby a motor in accordance with the drive of the hydraulic pump by anengine.

SUMMARY OF INVENTION

In this mixer drum driving device, the mixer drum is set in positiverotation and negative rotation by driving the auxiliary hydraulic pumpby the motor, and the mixing, pouring and discharging of the freshconcrete carried in the mixer drum are all performed by the motor. Thus,an inverter necessary to drive the motor is necessary and, in addition,a high-output motor needs to be used, leading to the enlargement of themotor and a power supply.

Since this increases the number of components for driving the motor andenlarges the motor, the power supply necessary to drive the motor andother components, mountability of the mixer drum onto a chassis isdeteriorated and the mixer truck becomes heavier. Thus, the loadcapacity of the mixer drum has to be reduced and the amount oftransportable fresh concrete decreases, thereby deterioratingtransportation efficiency. Further, since transportation efficiency isdeteriorated, the amount of fuel consumption increases by that much.

An object of the present invention is to provide a mixer drum drivingdevice capable of driving a mixer drum by a motor without reducingtransportation efficiency.

According to one aspect of the present invention, a mixer drum drivingdevice comprises a mixer drum rotatably mounted on a chassis of a mixertruck; a hydraulic motor for driving and rotating the mixer drum; ahydraulic pump for supplying pressure oil to the hydraulic motor bybeing driven by the power of an engine of the mixer truck; an auxiliaryhydraulic pump for rotating the mixer drum for mixing by supplyingpressure oil to the hydraulic motor independently of the hydraulic pump;and a direct-current brush motor for driving and rotating the auxiliaryhydraulic pump, wherein the auxiliary hydraulic pump is driven androtated by the direct-current brush motor when the engine stops duringthe rotation of the mixer drum for mixing.

Embodiments of the present invention and advantages thereof aredescribed in detail below with reference to the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing a mixer drum driving device in the presentembodiment.

FIG. 2 is a side view of a mixer truck carrying a mixer drum on achassis.

FIG. 3 is a rear view of the mixer drum mounted on the chassis of themixer truck.

FIG. 4 is a diagram showing a mixer drum driving device in anotherembodiment.

DESCRIPTION OF EMBODIMENTS

Hereinafter, an embodiment of the present invention is described withreference to the drawings.

As shown in FIG. 1 and FIG. 3, a mixer drum driving device S in thepresent embodiment includes a mixer drum M rotatably mounted on achassis C of a mixer truck V, a hydraulic motor 3 for driving androtating the mixer drum M, a hydraulic pump 4 for supplying pressure oilto the hydraulic motor 3 by being driven by the power of an engine E ofthe mixer truck V, an auxiliary hydraulic pump 5 capable of supplyingpressure oil to the hydraulic motor 3 independently of the hydraulicpump 4 to rotate the mixer drum M for mixing, a direct-current brushmotor 6 for driving and rotating the auxiliary hydraulic pump 5 and acontroller 7 for controlling the direct-current brush motor 6.

The mixer truck V includes legs T mounted on the chassis C and a pair ofrollers R, R rotatably equipped in the legs T and carries the mixer drumM and the hydraulic motor 3, the hydraulic pump 4, the auxiliaryhydraulic pump 5, the direct-current brush motor 6 and the controller 7necessary to drive and rotate the mixer drum M on the chassis C.

The mixer drum M includes a drum shell 1 in the form of a bottomed tubewith an open rear end and coupled to the hydraulic motor 3 at a shaftcenter portion of a bottom portion serving as a front end, and a rollerring 2 provided on the outer periphery of the rear end side of the drumshell 1. As shown in FIG. 2 and FIG. 3, the mixer drum M is rotatablymounted on the chassis C to support the roller ring 2 from below by therollers R, R. Furthermore, the mixer drum M is mounted on the chassis Cin a forward inclined posture with the rear end side lifted up.

A plurality of spiral blades is provided on an inner peripheral side ofthe drum shell 1. By these blades, a load such as fresh concrete carriedin the mixer drum M can be mixed while being moved to an inner side ifthe mixer drum M is set in positive rotation, whereas the load can bemoved to the rear end side and discharged from the mixer drum M if themixer drum M is set in negative rotation. Furthermore, in pouring thefresh concrete into the mixer drum M, the mixer drum M is set inpositive rotation at a higher speed than in rotation for mixing.

Accordingly, there are three rotation modes of the mixer drum M: apouring mode utilized in pouring the load, a mixing mode utilized inmixing the load and a discharging mode utilized in discharging the load.In the mixing mode, the mixer drum M is set in positive rotation at sucha low speed as to be able to prevent the solidification of the freshconcrete, e.g. at 1 to 2 rpm to suppress an increase in slump valuewhile preventing the solidification of the fresh concrete.

Although a case where the fresh concrete as the load is transported froma concrete plant to a placement site is described as an example in thepresent embodiment, the present embodiment is applicable also in a casewhere cleaning water is poured into the mixer drum M after freshconcrete is discharged at a placement site and the mixer truck returnsto a concrete plant while being cleaned. In this case, the cleaningwater is a load.

The hydraulic motor 3 is set to be capable of bidirectional rotation andconnected to the hydraulic pump 4 via a looped pipe line 8. Thehydraulic motor 3 is coupled to the mixer drum M and sets the mixer drumM in positive rotation in the case of positive rotation while settingthe mixer drum M in negative rotation in the case of negative rotation.It should be noted that a reduction gear may be interposed between thehydraulic motor 3 and the mixer drum M.

The hydraulic pump 4 is a one-way discharge type hydraulic pump which isprovided at an intermediate position of the pipe line 8 and dischargespressure oil toward the hydraulic motor 3, and a variable capacity typehydraulic pump such as a piston pump. The hydraulic pump 4 is coupled tothe engine E of the mixer truck V via a PTO 9 and driven and rotated bythe power of the engine E.

To bidirectionally rotate the hydraulic motor 3 by the hydraulic pump 4that discharges the pressure oil in one direction, a switching valve 20is provided at an intermediate position of the pipe line 8. Theswitching valve 20 is a 4-port 3-position switching valve having threepositions: a position 20 a where the pressure oil of the hydraulic pump4 is fed to the hydraulic motor 3 to set the hydraulic motor 3 inpositive rotation, a position 20 b where the pressure oil of thehydraulic pump 4 is fed to the hydraulic motor 3 to set the hydraulicmotor 3 in negative rotation and a position 20 c where the hydraulicmotor 3 and the hydraulic pump 4 are disconnected.

It should be noted that the hydraulic pump 4 may be set to be of abidirectional discharge type. In this case, the hydraulic pump 4 and thehydraulic motor 3 are connected by a looped pipe line without providingthe switching valve 20 and the hydraulic motor 3 can be driven androtated in both positive and negative directions by switching adischarging direction of the hydraulic pump 4.

The auxiliary hydraulic pump 5 is provided at an intermediate positionof an auxiliary supply pipe line 11 connecting a side of the pipe line 8upstream of the hydraulic motor 3 when the hydraulic motor 3 is inpositive rotation and a tank 10. Thus, the auxiliary hydraulic pump 5can suck oil from the tank 10 and supply pressure oil to an upstreamside when the hydraulic motor 3 is in positive rotation. It should benoted that a side of the pipe line 8 downstream of the hydraulic motor 3when the hydraulic motor 3 is in positive rotation is connected to thetank 10 via an auxiliary discharge pipe line 12.

The direct-current brush motor 6 is connected to a power supply Bat torotate only in one direction. The auxiliary hydraulic pump 5 is drivenand rotated by the direct-current brush motor 6 and sucks oil from thetank 10 and discharges it toward the hydraulic motor 3. Further, aswitching valve 13 is provided at intermediate positions of theauxiliary supply pipe line 11 and the auxiliary discharge pipe line 12.The switching valve 13 is a 4-port 2-position switching valve having twopositions: a position 13 a where pressure oil discharged from theauxiliary hydraulic pump 5 is fed to the upstream side of the pipe line8 when the hydraulic motor 3 is in positive rotation via the auxiliarysupply pipe line 11 to set the hydraulic motor 3 in positive rotationand the downstream side of the pipe line 8 when the hydraulic motor 3 isin positive rotation is connected to the tank 10 via the auxiliarydischarge pipe line 12 and a position 13 b where the auxiliary hydraulicpump 5 is connected to the tank 10 and the pressure oil discharged fromthe auxiliary hydraulic pump 5 is returned to the tank 10 without viathe hydraulic motor 3.

When the direct-current brush motor 6 is driven by power supply from thepower supply Bat, the auxiliary hydraulic pump 5 rotates to suck the oilfrom the tank 10 and discharge the pressure oil. If the switching valve13 is at the position 13 a where the pressure oil is supplied to thehydraulic motor 3, the pressure oil discharged from the auxiliaryhydraulic pump 5 is supplied to the hydraulic motor 3 and the hydraulicmotor 3 is set in positive rotation.

The mixer drum driving device S in the present embodiment includes aselect lever 14 capable of selecting the rotation mode of the mixer drumM. When an operator of the mixer truck V operates the select lever 14,the mixer drum M rotates in the selected rotation mode. Specifically,the operator can select any one of the pouring mode for setting themixer drum M in positive rotation at a high speed, the mixing mode forsetting the mixer drum M in positive rotation at a low speed and thedischarging mode for setting the mixer drum M in negative rotation at ahigh speed by operating the select lever 14 in a direction of abroken-line arrow in FIG. 1.

The select lever 14 is coupled to an unillustrated governor of theengine E via a link or the like and can rotate the mixer drum M at ahigh speed by increasing the rotation speed of the engine E in thepouring mode and the discharging mode. Further, the select lever 14 candrive an unillustrated actuator such as a solenoid for switching theswitching valve 20. In the pouring mode and the mixing mode, theswitching valve 20 is switched to the position 20 a where the pressureoil is so supplied as to set the hydraulic motor 3 in positive rotation.In the discharging mode, the switching valve 20 is switched to theposition 20 b where the pressure oil is so supplied as to set thehydraulic motor 3 in negative rotation.

It should be noted that, instead of switching the switching valve 20 bythe solenoid, the select lever 14 and the switching valve 20 may becoupled via a link or the like and the switching valve 20 may beswitched by the operation of the select lever 14.

When the select lever 14 is set in the mixing mode, an angle ofinclination of the hydraulic pump 4 is automatically adjusted so thatthe discharge flow rate of the hydraulic pump 4 is constant regardlessof the rotation speed of the engine E. By the action of this adjustingmechanism, the mixer drum M is set in positive rotation at a constantspeed regardless of the rotation speed of the engine E.

The controller 7 is connected to a rotation speed sensor 18 fordetecting an engine rotation speed, a proximity switch 15 for outputtingan ON-signal when the select lever 14 is located to set the mixing modeand a switch 17 provided at an intermediate position of a power-supplyline 16 connecting the power supply Bat and the direct-current brushmotor 6. When the select lever 14 is located to set the mixing mode, anON-signal of the proximity switch 15 is input to the controller 7. Thecontroller 7 recognizes that the mixer drum M is being rotated formixing by the ON-signal of the proximity switch 15.

Further, the controller 7 recognizes that the engine E is stopped whenthe engine rotation speed becomes 0. As just described, the controller 7judges that the mixer drum M is being rotated for mixing and that theengine E is in a stopped state from the ON-signal of the proximityswitch 15 and the engine rotation speed detected by the rotation speedsensor 18. It should be noted that the controller 7 only has to beconfigured so as to be able to make the above judgment and turn on andoff the switch 17 by outputting signals to the switch 17. Further, thecontroller 7 may receive an engine rotation pulse signal or an idlingstop signal and judges whether or not the engine is stopped, utilizingat least one of these signals.

The controller 7 performs an ON-operation of the switch 17 when themixer drum M is being rotated for mixing and the engine E is in thestopped state. This causes power to be supplied from the power supplyBat to the direct-current brush motor 6, whereby the auxiliary hydraulicpump 5 is driven. Thus, the pressure oil supplied from the auxiliaryhydraulic pump 5 instead of the hydraulic pump 4 drives the hydraulicmotor 3 and the mixer drum M seamlessly continues rotation for mixing.

Specifically, the mixer drum driving device S drives and rotates themixer drum M only by the direct-current brush motor 6 to rotate themixer drum M for mixing when the engine E is set in the stopped state.The controller 7 switches the switching valves 13, 20 in addition to theON-operation of the switch 17. The controller 7 is connected tounillustrated actuators such as solenoids for driving and switching theswitching valves 13, 20. In the case of driving and rotating the mixerdrum M only by the direct-current brush motor 6, the switching valve 13is switched to the position 13 a where the pressure oil discharged fromthe auxiliary hydraulic pump 5 is fed to the hydraulic motor 3 and theswitching valve 20 is switched to the position 20 c where the hydraulicpump 4 is disconnected from the hydraulic motor 3 on a hydrauliccircuit. That is, the hydraulic pump 4 and the hydraulic motor 3 aredisconnected.

By disconnecting the hydraulic pump 4 from the hydraulic motor 3 in thisway, the pressure oil of the auxiliary hydraulic pump 5 can be preventedfrom flowing toward the hydraulic pump 4 and escaping to the tank 10 viathe hydraulic pump 4 and the mixer drum M can be efficiently driven androtated.

Further, when the engine E is restarted after being stopped during therotation of the mixer drum M for mixing, the controller 7 turns off theswitch 17 to stop the direct-current brush motor 6. Then, the hydraulicmotor 3 is driven by the pressure oil supplied from the hydraulic pump 4driven by the engine E instead of the pressure oil from the auxiliaryhydraulic pump 5. In this way, the rotation of the mixer drum M formixing is seamlessly continued.

In the case of switching the drive source for the mixer drum M from thedirect-current brush motor 6 to the engine E, the controller 7 switchesthe switching valve 20 to the position 20 a where the pressure oil isfed to set the hydraulic motor 3 in positive rotation and switches theswitching valve 13 to the position 13 b where the auxiliary hydraulicpump 5 is disconnected from the hydraulic motor 3 on the hydrauliccircuit. This causes the auxiliary hydraulic pump 5 and the hydraulicmotor 3 to be disconnected. By disconnecting the auxiliary hydraulicpump 5 from the hydraulic motor 3 in this way, the pressure oil of thehydraulic pump 4 can be prevented from flowing toward the auxiliaryhydraulic pump 5 and escaping to the tank 10 via the auxiliary hydraulicpump 5.

The mixer drum driving device S of the present embodiment mixes thefresh concrete in the mixer drum M using the direct-current brush motor6 only in the state where the engine E is stopped. Since the drum isrotated by the drive of the auxiliary hydraulic pump 5 only for mixing,the rotation speed of the auxiliary hydraulic pump 5 may beunidirectional and constant and the rotation speed of the motor 6 mayalso be constant.

By doing so, a maximum torque required for a motor can be smaller ascompared with the case where all the functions of pouring, mixing anddischarging of the mixer drum M are performed only by a motor. Thus, thedirect-current brush motor 6 and the power supply Bat can beminiaturized without requiring an inverter.

Accordingly, since the direct-current brush motor 6 and the power supplyBat to be mounted on the chassis C of the mixer truck V can beminiaturized and an inverter is not necessary, a sufficient capacity canbe ensured for the mixer drum M. Furthermore, since the mixer truck Vbecomes lighter in weight, fresh concrete load capacity can be increasedby an amount corresponding to a reduction in weight. Thus, the mixerdrum M can be driven by a motor without leading to a reduction intransportation efficiency due to a reduction in the amount oftransportation of fresh concrete.

Furthermore, since transportation efficiency is not reduced, the amountof fuel consumption of the mixer truck V can be reduced. Further, sincethe direct-current brush motor 6 and the power supply Bat to be mountedon the chassis C of the mixer truck V can be miniaturized and aninverter is not necessary, manufacturing cost can be reduced.

Furthermore, since a drive system for the mixer drum M is composed oftwo systems of the engine E and the direct-current brush motor 6, evenif a certain trouble occurs in either one of the systems and the mixerdrum M cannot be rotated, the mixer drum M can be driven and rotated bythe other system.

Further, the configuration of the hydraulic circuit may be as shown inFIG. 4. It should be noted that the configuration of the mixer drumdriving device other than the hydraulic circuit such as the engine E,the controller 7, the switch 17, the power supply Bat and the mixer drumM is not shown in FIG. 4.

The hydraulic circuit shown in FIG. 4 is configured to include anauxiliary supply pipe line 21 connecting the side of the pipe line 8upstream of the hydraulic motor 3 when the hydraulic motor 3 is inpositive rotation and the auxiliary hydraulic pump 5, the tank 10, anauxiliary discharge pipe line 22 connecting the side of the pipe line 8downstream of the hydraulic motor 3 when the hydraulic motor 3 is inpositive rotation and the tank 10, a check valve 23 provided at anintermediate position of the auxiliary supply pipe line 21 forpermitting only a flow of the pressure oil from the auxiliary hydraulicpump 5 toward the pipe line 8, and an on-off valve 24 provided at anintermediate position of the auxiliary discharge pipe line 22 foropening and closing the auxiliary discharge pipe line 22.

When the mixer drum M is being rotated for mixing and the engine E is inthe stopped state, power is supplied to the direct-current brush motor 6and the auxiliary hydraulic pump 5 is driven and the on-off valve 24 isopened to allow the side of the pipe line 8 downstream of the hydraulicmotor 3 when the hydraulic motor 3 is in positive rotation tocommunicate with the tank 10 via the auxiliary discharge pipe line 22.Then, the pressure oil discharged from the auxiliary hydraulic pump 5 issupplied to the hydraulic motor 3 to set the hydraulic motor 3 inpositive rotation and the pressure oil discharged from the hydraulicmotor 3 is collected to the tank 10.

By configuring the mixer drum driving device in this way, the auxiliarydischarge pipe line 22 is reliably blocked by the on-off valve 24 andthe auxiliary supply pipe line 21 is reliably blocked by the check valve23 while the engine E is driven and the hydraulic motor 3 is driven bythe hydraulic pump 4. Thus, it can be reliably prevented that oil leaksfrom the pipe line 8 side to the tank 10 and becomes insufficient in thepipe line 8 while the hydraulic motor 3 is driven by the hydraulic pump4.

Further, a relief valve 25 for allowing the pressure oil of theauxiliary hydraulic pump 5 to escape to the tank 10 is provided betweenthe auxiliary supply pipe line 21 and the auxiliary discharge pipe line22. Thus, an upper limit of the discharge pressure of the auxiliaryhydraulic pump 5 is limited to a valve opening pressure of the reliefvalve 25, whereby an excessive pressure does not act on the hydrauliccircuit in the mixer drum driving device and this hydraulic circuit andhydraulic devices provided in the hydraulic circuit such as thehydraulic motor 3 and the auxiliary hydraulic pump 5 can be protected.

It should be noted that although the controller 7 recognizes that themixer drum M is being rotated for mixing based on the ON-signal from theproximity switch 15 that outputs the ON-signal upon the approach of theselect lever 14 when the select lever 14 is located at the position toinstruct the mixing mode in the present embodiment, the selection of themixing mode may be recognized using another sensor. It is also possibleto use an operation button or a selection switch instead of the selectlever 14.

Furthermore, a weight sensor for detecting the weight of the mixer drumM may be provided on the leg T for supporting the mixer drum M and thedirect-current brush motor 6 may be driven to drive and rotate the mixerdrum M when the mixer drum M is being rotated for mixing and the engineE is in the stopped state and, in addition, the weight detected by theweight sensor is not smaller than a predetermined weight. If a statewhere the load such as fresh concrete or cleaning water is carried inthe mixer drum M and a state where no load is carried are compared, themixer drum M carrying the load is heavier. Thus, by setting thepredetermined weight at a weight exceeding that of the empty mixer drumM, whether or not any load such as fresh concrete is carried in themixer drum M can be judged.

In this case, even if it is attempted to rotate the empty mixer drum Mfor mixing due to an operation error of the operator, useless waste ofpower due to the drive of the direct-current brush motor 6 while theengine is stopped can be prevented since the weight detected by theweight sensor is below the predetermined weight.

Further, judgment as to whether or not any load such as fresh concreteor cleaning water is carried in the mixer drum M can be also made bydetecting a pressure upstream of the hydraulic motor 3 in the pipe line8 along a flowing direction of the pressure oil in setting the hydraulicmotor 3 in positive rotation. If a state where the load is carried inthe mixer drum M and a state where no load is carried are compared, alarger torque is required in driving and rotating the mixer drum M inthe state where the load is carried. Thus, the pressure of the pressureoil supplied to the hydraulic motor 3 is increased by that much.Therefore, by setting the pressure of the pressure oil when thehydraulic motor 3 is driven in the state where the load is carried inthe mixer drum M as a predetermined pressure, whether or not any load iscarried in the mixer drum M can be judged.

Further, instead of a pressure sensor, a pressure switch which is turnedon at the above predetermined pressure and outputs an ON-signal to thecontroller 7 may be provided and the controller 7 may judge that theload is carried in the mixer drum M when the ON-signal is input.

The embodiments of the present invention described above are merelyillustration of some application examples of the present invention andnot of the nature to limit the technical scope of the present inventionto the specific constructions of the above embodiments.

The present application claims a priority based on Japanese PatentApplication No. 2011-065503 filed with the Japan Patent Office on Mar.24, 2011, all the contents of which are hereby incorporated byreference.

1. A mixer drum driving device, comprising: a mixer drum rotatablymounted on a chassis of a mixer truck; a hydraulic motor for driving androtating the mixer drum; a hydraulic pump for supplying pressure oil tothe hydraulic motor by being driven by the power of an engine of themixer truck; an auxiliary hydraulic pump for rotating the mixer drum formixing by supplying pressure oil to the hydraulic motor independently ofthe hydraulic pump; and a direct-current brush motor for driving androtating the auxiliary hydraulic pump, wherein the auxiliary hydraulicpump is driven and rotated by the direct-current brush motor when theengine stops during the rotation of the mixer drum for mixing.
 2. Themixer drum driving device according to claim 1, comprising: a loopedpipe line connecting the hydraulic pump and the hydraulic motor; anauxiliary supply pipe line connecting a side of the pipe line upstreamof the hydraulic motor when the hydraulic motor is in positive rotationand the auxiliary hydraulic pump; a tank; an auxiliary discharge pipeline connecting a side of the pipe line downstream of the hydraulicmotor when the hydraulic motor is in positive rotation and the tank; anda switching valve having a position where the auxiliary hydraulic pumpis connected to the pipe line via the auxiliary supply pipe line and thepipe line is connected to the tank via the auxiliary discharge pipe lineand a position where the auxiliary hydraulic pump is connected to thetank to return pressure oil discharged from the auxiliary hydraulic pumpto the tank without via the hydraulic motor.
 3. The mixer drum drivingdevice according to claim 1, comprising: a looped pipe line connectingthe hydraulic pump and the hydraulic motor; an auxiliary supply pipeline connecting a side of the pipe line upstream of the hydraulic motorwhen the hydraulic motor is in positive rotation and the auxiliaryhydraulic pump; a tank; an auxiliary discharge pipe line connecting aside of the pipe line downstream of the hydraulic motor when thehydraulic motor is in positive rotation and the tank; a check valveprovided at an intermediate position of the auxiliary supply pipe linefor permitting only a flow of the pressure oil from the auxiliaryhydraulic pump toward the pipe line; and an on-off valve provided at anintermediate position of the auxiliary discharge pipe line for openingand closing the auxiliary discharge pipe line.
 4. The mixer drum drivingdevice according to claim 3, comprising: a relief valve for allowing thepressure oil of the auxiliary hydraulic pump to escape to the tank.